From: Jakma, Paul (Paul.Jakma at domain compaq.com)
Date: Fri 04 Feb 2000 - 11:17:13 GMT
> Kenn Humborg writes:
> On a similar note, what is the difference between Ultra Wide
> and Ultra 2 Wide? And what is LVD?
> All I know is that wide equates to 68-pin connetors ...
Ultra uses absolute signalling, ie logic 1 = x Volts on the line (x = 3
iirc), logic 0 = 0V.
that kind of signalling becomes more and more prone to distortion as the
clock speed goes up. So the new industry standard:
Ultra2 (aka LVD -> low voltage differential)
Uses differential signalling. This uses 2 signalling wires and logic level
is determined by the polarity of one compared to the other. Eg Logic 1 when
+ / - and logic 0 when - / +.
Measuring polarity reversal is much easier to do reliably than measuring an
absolute value reliably, so with LVD cable lengths can be much longer, and
clock can be much higher. Also, as suggested by the LV bit, it uses lower
voltage signals to represent + and -, (1 to 1.5V absolute iirc).
Eg Ultra clocks at 20MHz and is very finicky about termination, cable
lengths and physical spacing of devices on the bus. Ultra2 clocks at 40MHz
yet is far less troublesome, and cable lengths are much longer. Ultra3
clocks at 80MHz. :)
The wide bit refers to the width of the bus. SCSI-2 had narrow (byte width
bus), and wide ( 2byte width bus ). AFAIK Ultra2 has dropped narrow support.
All new Ultra2/Ultra3 devices will be 16bit/wide.
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